1. Field of the Invention
The present invention relates to an X-ray mask which is used in the manufacture of microdevices such as semiconductor integrated circuits to transfer a micropattern onto a wafer, and an X-ray exposure method and apparatus using the same.
2. Related Background Art
Semiconductor integrated circuits continue to be minutely downsized in their feature size year by year. Along with this trend, an exposure apparatus for transferring a circuit pattern onto a wafer is required to transfer increasingly minute circuit patterns. For this reason, the wavelength of exposure light used in exposure apparatuses is gradually becoming shorter, and recently, an exposure apparatus using soft X-rays as a transfer means for the most elaborate circuit patterns has been examined.
Upon transferring the circuit pattern of a semiconductor integrated circuit by exposure, contamination due to foreign matter attached to the exposure mask is a very serious problem. If foreign matter becomes attached to a mask, it is transferred onto the wafer and becomes a defect in the circuit pattern. For this reason, attachment of foreign matter must be prevented as much as possible. In particular, in exposure using soft X-rays, since the transmittance of X-rays through various substances is very low, X-rays cannot be transmitted through even very small foreign matter, thus causing defects.
As the circuit pattern is more downsized, it becomes more difficult to remove foreign matter on the mask or to repair the mask. In X-ray masks, such difficulties are pronounced due to their structures.
The X-ray masks are roughly classified into a transmission type mask and a reflection type mask. In a transmission type mask, since a mask pattern (circuit pattern) is formed on a self-supported film, the mask must be repaired to prevent changes in stress of the self-supported film. On the other hand, in a reflection type mask using a multi-layered film reflection mirror, since the reflection layer has a multilayered structure, the mask must be repaired to preclude any influence on the multilayered structure. These techniques, however, are very difficult to achieve. Hence, attachment of foreign matter must be avoided more carefully in the X-ray masks than the masks used in exposure apparatuses using visible light and ultraviolet rays.
In exposure apparatuses using visible light and ultraviolet rays, a pellicle consisting of an organic thin film such as nitrocellulose, parylene, or the like is often set on the mask so as to prevent attachment of foreign matter. Since the pellicle is set therein, foreign matter can be prevented from becoming attached to the mask and, hence, contamination can be avoided.
In particular, in a projection exposure type exposure apparatus, when the pellicle is set to fall outside the field depth of an imaging optical system, even when foreign matter becomes attached onto the pellicle, it is never transferred onto the wafer, and any foreign matter attached onto the pellicle can be easily removed. For this reason, the yield of transferred patterns can be improved greatly. Also, once mask inspection is done upon setting the pellicle, foreign matter inspection on the pellicle need only be performed from then on.
Therefore, in some examples (Japanese Patent Publication No. 5-88534, Japanese Laid-Open Patent Application No. 5-150445), an X-ray transmission thin film such as a diamond film, an organic thin film, or the like, a so-called pellicle, is set on an X-ray mask as well.